The present invention concerns isolators utilizing the propagation of non-reciprocal surface electromagnetic waves in a gyromagnetic medium. Such devices have already formed the subject of publications; there will be mentioned inter alia U.S. Pat. No. 3,845,413 filed on the 23rd Oct., 1973 and assigned to the same Assignor. Articles have been published in Cables et Transmissions (France), October 1973, pages 416 to 435, and in Transactions on Magnetics of the Institute of Electrical and Electronic Engineers (United States) - Vol Mag 11 - No. 5 - September 1975, page 1276.
The articles bear more particularly on the analysis of the surface non-reciprocal propagation modes and of the parasitic modes (reciprocal volume or surface propagation modes), which can be excited at frequencies within the band to be transmitted and are propagated in the gyromagnetic material simultaneously with the desired non-reciprocal surface mode. The parasitic modes which are closest to the operating mode (which is termed the "dynamic mode" in the cited articles) are volume modes. The present invention relates essentially to means for reducing the proportion of the input energy which is converted into a parasitic wave, which energy is taken from that which is propagated in the dynamic mode. In other words, the invention has for its object to reduce the insertion losses of the devices and at the same time the standing wave ratio which they introduce into the equipment in which they are fitted while approaching as closely as possible to a monomode propagation. The invention also relates to means for selectively attenuating the parasitic waves so as to reduce the transmitted parasitic energy, which results in a reduction of the amplitude variations in the bandwidth and provides an increase of the bandwidth at a preset insertion loss, or a reduction of the insertion loss at a preset bandwidth.